The inventive concepts described herein relate to a power amplifier and, more particularly, to a linearized power amplifier.
In carrying out power amplification, a power amplifier normally utilizes nonlinear elements such as transistors, diodes, and other nonlinear semiconductor devices. As a result, a nonlinear distortion may occur to an output signal of the power amplifier. For instance, as the magnitude of an input signal increases, the ratio between the magnitude of the output signal and that of the input signal, i.e., a gain of the power amplifier, may be varied. This phenomenon is known as amplitude modulation-amplitude modulation distortion (hereinafter referred to as AM-AM distortion). Similarly, as the magnitude of an input signal increases, a phase shift of the output signal may also occur. This phenomenon is known as amplitude modulation-phase modulation distortion (hereinafter referred to as AM-PM distortion). Such distortions may deteriorate the reliability of the output signal, and the efficiency of the power amplifier.
In view of the aforementioned problems, efforts have been made to attenuate the nonlinearity of a power amplifier. One approach is to provide an envelope-based bias circuit. As illustrated in FIG. 10A, the envelope-based bias circuit 500 includes an envelope detector 510 and a replica 520 of a power amplification stage 530. The envelope detector 510 generates an envelope signal from an input signal. The envelope signal is used to control the replica 520 to produce an analog output signal which represents an inverse of an AM-AM distortion of the power amplification stage 530. Then, the envelope-based bias circuit 500 biases the input signal by using the analog output signal to compensate for the AM-AM distortion of the power amplification stage 530. However, the envelope-based bias circuit 500 is incapable of satisfactorily compensating for an AM-PM distortion of the power amplification stage 530.
As an alternative to the envelope-based bias circuit, there has been proposed a closed-loop linearization circuit 550, illustrated in FIG. 10B. The closed-loop linearization circuit 550 includes a difference unit 560, a processor 570, and a linearizer 580. The difference unit 560 subtracts an input signal from an output signal to obtain sideband information of the output signal that is indicative of the nonlinear characteristics of a power amplification stage 590. The sideband information is then delivered to the processor 570. The processor 570 uses the sideband information to adjust one or more elements within the linearizer 580 so that the closed-loop linearization circuit 550 can dynamically respond to the nonlinear characteristics of the power amplification stage 590. In this way, the closed-loop linearization circuit 550 can compensate for the AM-AM distortion as well as the AM-PM distortion of the power amplification stage 590. However, the closed-loop linearization circuit 550 has a complicated structure, and consumes high direct current (DC) power, which results in an inefficient space utilization and power consumption.